Digestion and Immunity

An In-Depth Look at Human Digestion

Human Digestion Human digestion starts at the mouth and ends at the anus. There are several mechanisms in the mouth that immediately begin to break down the foods we eat and prepare them for the rest of the digestive process. Through the process of mastication, or chewing, we are physically breaking the food into smaller pieces, which will help it travel more easily through the rest of the digestive system as well as increase the surface area available for the chemical agents in our body to bind to and break down the food. Glands under the tongue secrete saliva, a mixture of water, mucus, proteins, mineral salts and the enzymes lingual lipase and salivary amylase. Saliva moistens the food and begins to break down the fats and carbohydrates while the teeth and tongue combine the food into a mushy ball. As we swallow, this ball, now referred to as a bolus, is pushed to the back of the oral cavity and into the esophagus. A flap of muscle known as the epiglottis closes over the trachea, or windpipe, preventing any swallowed solids or liquids from entering the lungs. Once in the esophagus, the bolus is moved into the stomach via a process called peristalsis, which is a downward wave of muscle contraction. Peristalsis continues through most of the digestive system and is the primary mechanism that moves foods through the digestive tract. This action is also referred to as a “housecleaning wave”. Where the esophagus attaches to the stomach, there is a valve called the lower esophageal sphincter that regulates the movement of material from the stomach. In normal circumstances, it stays closed except when allowing a bolus to pass from the esophagus into the stomach. It typically takes roughly 10 seconds for food to pass from the top of the esophagus to the stomach. Stomach The stomach is essentially a bag made of three layers of muscle. Because of this muscled composition, the stomach is capable of contraction and expansion. It can expand to accommodate roughly a liter of food and liquid at once before any distention pressure is felt. It is also this ability to expand and contract that allows the stomach to mix, grind and churn the bolus. Additionally, 1.2 to 1.5 liters of gastric juice is secreted per day into the stomach. Gastric juice is a mixture of water, hydrochloric acid, electrolytes (sodium, potassium, calcium, phosphate, sulfate, and bicarbonate), mucus and enzymes. This juice is highly acidic due to its hydrochloric acid content and contains enzymes, both to break the bolus down further and make it more soluble in preparation for absorption to occur in the small intestine. At this point, the semisolid mixture created from the bolus is called chyme. While in the stomach, some absorption can occur though not very much. Small amounts of fluid (such as water and alcohol) can be absorbed from the stomach as well as some simple sugars like glucose and some amino acids. Many pharmacological agents are absorbed here as well. The time that the chyme stays in the stomach depends on the chemical and physical composition of the meal as well as the specific physiology of the individual. Fluids empty the most rapidly followed by carbohydrate, protein, and fat in that order. After an average of 2 to 4 hours in the stomach, when the food particles in the chyme have been reduced sufficiently in size and are at the appropriate level of solubility, the chyme will move through the valve at the base of the stomach, called the pyloric sphincter into the first section of the small intestine called the duodenum. (doo-oh-dee’-num) Small Intestine The small intestine is roughly 20 to 25 feet long and about 2 inches in diameter. It is the longest section of the digestive system and is divided into three sections. The first section, where the stomach meets the small intestine is called the duodenum. The duodenum is roughly 9 to 11 inches in length and contains the duodenal papilla where pancreatic juice and bile flows into the small intestine. When the chyme enters the duodenum, special cells in the walls produce the hormones secretin and cholecystokinin. These hormones signal for the pancreas to deliver pancreatic juice, which contains the enzymes trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases, and amylase as well as bicarbonate. The bicarbonate concentration neutralizes whatever stomach acid comes with the chyme so that the enzymes are able to work and the small intestinal wall is not damaged by the hydrochloric acid. [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”] Cholecystokinin and secretin also signal for the sphincter of Oddi, a valve at the base of the common bile duct to relax, releasing bile into the duodenum. Bile is a brownish yellow liquid that is continuously produced by the liver. It is composed of water, bicarbonate, phospholipids, bile salts, emulsifying agents, cholesterol, and bile pigments. It is stored and concentrated in the gallbladder until release into the duodenum for digestion. Bile serves important functions in fat digestion, coating fat particles from food and allowing greater surface area for pancreatic lipase to work. The combination of enzymes and bile acids serves to further reduce the particle size and increase the solubility of the chyme. This allows for the nutrients within the food to pass through the walls of the small intestine and be carried throughout the body for use. Enzymes within the walls of the small intestine carry out any final breakdown of nutrients as the chyme travels toward the large intestine. The remaining length of the small intestine is divided into the jejunum and ileum. These sections are much longer than the duodenum. The jejunum is the middle section and is roughly 8 feet long while the ileum, the final section of the small intestine, is nearly 12 feet long. The chyme travels along the length of these sections and the useable nutrients